Add-drop multiplexers are used, for instance, in nodes of a wavelength division multiplexing communication network, to extract a channel from the multiplexed stream, letting the remaining channels pass through unaltered, and to add a new channel to the multiplexed stream. In practice, it is advantageous that the dropped channel and the added channel are allocated the same wavelength position to simplify network management.
Typically, add-drop multiplexers are four-port devices, with two ports for the input/output of the multiplexed stream and two ports for the input/output of the individual channel. For convenience, the input and output ports for the multiplexed stream are referred to as the express input and output ports, respectively, and the input and output ports for the new and extracted channels are referred to as the “add” and the “drop” ports, respectively.
Conventionally, prior art add-drop multiplexers have used fixed or tunable filters to provide the necessary wavelength selectivity for the add/drop function, e.g., fibre Bragg gratings, non-absorbing interference filters, etc. Typically, when these prior art systems are configurable between express passthrough modes of operation and add/drop modes of operation, the design relies upon physical movement of the filter in and out of the optical path.
In U.S. Pat. Nos. 5,606,439 and 5,694,233, Kuang-Yi Wu discloses an add/drop optical filter including a plurality of birefringent elements and Faraday rotator for switching channels of a multiplexed beam of light between first and second output ports. However, neither of these patents teaches a four port add/drop device that is configurable between express passthrough modes of operation and add/drop modes of operation.
It is an object of this invention to provide an add/drop multiplexer that offers configurabilty between passthrough and add/drop states with non-moving parts.
It is another object of this invention to provide a configurable four port add/drop device that uses a controllable polarisation switch as the switching mechanism.